In general, magnetic resonance imaging (MRI) examinations are based on the interactions among a primary magnetic field, a radiofrequency (RF) magnetic field and time varying magnetic gradient fields with gyromagnetic material having nuclear spins within a subject of interest, such as a patient. Certain gyromagnetic materials, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of spins of these nuclei can be influenced by manipulation of the fields to produce RF signals that can be detected, processed, and used to reconstruct a useful image.
The magnetic fields used to generate images in MRI systems include a highly uniform, static magnetic field that is produced by a primary magnet. A series of gradient fields are produced by a set of gradient coils located around the subject. The gradient fields encode positions of individual plane or volume elements (pixels or voxels) in two or three dimensions. An RF coil is employed to produce an RF magnetic field. This RF magnetic field perturbs the spins of some of the gyromagnetic nuclei from their equilibrium directions, causing the spins to precess around the axis of their equilibrium magnetization. During this precession, RF fields are emitted by the spinning, precessing nuclei and are detected by either the same transmitting RF coil, or by one or more separate coils. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one or more algorithms to reconstruct a useful image.
The features used to detect the emitted RF fields, such as an array of receiving coils, may not have similar sensitivities to the emitted RF fields. Thus, the different sensitivities of the detection elements can result in variations in the intensity of a reconstructed MR image. Accordingly, techniques have been developed for image processing that enable the correction of MR images having variable intensities resulting from coil sensitivity differences. Some such techniques utilize an additional pre-acquisition scan, a so-called “scout” scan, using a different set of coils (e.g., a body coil) than the coils (e.g., a spine coil array) that will produce the desired image. Other techniques may utilize RF field estimation or phantom calibration to determine the sensitivities of the detection elements.
However, current techniques employing such methods are often inadequate or are subject to further improvement. For example, the above techniques may utilize at least one scan in addition to the scan(s) used to acquire an image of the subject of interest, which increases overall scan time and reduces throughput. Further, such techniques may require access to the MR system utilized to acquire the scans of interest to obtain system-specific data, such as the particular location of the detection elements (e.g., receiving coils) within the MR system. In other words, system-specific calibrations along with associated scans may be implemented in such techniques. This can also hinder throughput and offsite post-acquisition processing. Accordingly, it is now recognized that a need exists for improved methods for data acquisition and reconstruction in magnetic resonance imaging techniques using coil arrays that may be subject to different sensitivities to RF fields.